1. Field of the Invention
The present invention relates to a differential amplifier, and more specifically to a variable gain differential amplifier.
2. Description of the Related Art
Hitherto, variable gain amplifiers have been used in communications applications. A variable gain amplifier amplifies an input signal to an output signal by a gain. The gain is variable by inputting a gain control signal from the outside. The variable gain amplifier can vary the gain in response to the gain control signal without interrupting the amplification.
Assuming that voltage amplitude of the input signal is kept constant, when the voltage gain is varied, voltage amplitude of the output signal changes in response to the voltage gain. If the peak level of the output signal is fixed to a reference potential, the DC level, corresponding to the center level, of the output signal moves downward when the voltage amplitude of the output signal increases. In order to stabilize the DC level, some variable gain amplifiers have been studied.
For example, Japanese Unexamined Patent Application Publication Nos. 61-219208, 62-245810, and 2012-28859 disclose amplifiers that have such a stabilization mechanism. A variable gain amplifier disclosed by Japanese Unexamined Patent Application Publication No. 61-219208 has two pairs of bipolar transistors, and adds collector current of one of the first paired transistors and collector current of one of the second paired transistors, and equidistributes the sum collector current to collector current of the other of the first paired transistors and collector current of the other of the second paired transistors. A variable gain amplifier disclosed by Japanese Unexamined Patent Application Publication No. 62-245810 has two differential amplifiers, and sum a current that does not flow through a resistive load of the first differential amplifier and a current that does not flow through a resistive load of the second differential amplifier to each other, and divide the sum of the currents into two output currents each flowing through the resistive loads. A variable gain differential amplifier disclosed by Japanese Unexamined Patent Application Publication No. 2012-28859 includes an adjustment circuit that includes two differential amplifiers and two current sources, and adjusts the respective DC levels of complementary output voltages in response to changes in voltage gain.
Variable gain amplifiers of the related art have complicated mechanisms for stabilizing the DC level of an output signal against changes in voltage gain. Such complicated mechanisms need large circuit size and large power consumption.